1. Field of the Invention
The present invention relates to auxiliary air supplying system for supplying auxiliary air to upstream of an exhaust emission control device in an exhaust system of an internal combustion engine, and to control methods and failure diagnostic methods thereof.
2. Related Background Art
As an exhaust emission control device of an internal combustion engine, there is a known system in which a catalytic converter is provided in the exhaust system to reduce CO, HC, NOx components, and others in the exhaust gas. Furthermore, there is the known technology of supplying auxiliary air through an auxiliary air inlet line provided with a control valve connected to an exhaust pipe, into the exhaust pipe to increase the oxygen content so as to induce secondary combustion of HC and CO in the exhaust gas, thereby facilitates cleaning-up the exhaust gas.
In such auxiliary air supplying system, if something unusual occurs in a component such as an air pump or the control valve, the emission control efficiency will decrease, so as to worsen the emission. Therefore, the unusual conditions must be evaluated in the early stage. The known techniques of detecting such unusual condition include the technologies disclosed in JP 09-21312 A and JP 09-125945 A.
The former describes the technology of disposing a pressure sensor between the air pump and the control valve on the auxiliary air inlet line and detecting the anomaly of the auxiliary air supplying system on the basis of detected pressure values. The latter describes the technology of disposing a pressure sensor on the auxiliary air inlet line and detecting the anomaly of the auxiliary air supplying system on the basis of detected pressure pulse width or the difference between maximum and minimum values thereof.
JP 06-17645 A discloses the technology of enabling switching between supply of auxiliary air to the exhaust port and supply of auxiliary air to upstream of the emission control device and temporarily communicating the exhaust port with the auxiliary air inlet line upstream of the emission control device at the time of halting the engine to discharge condensed water in the passage, thereby preventing blockage of the auxiliary air inlet line due to freezing.